igneous
Dark gray fine-grained rock fragment
Likely basalt (possibly andesite); exact identification is limited by the photograph
AI-generated identification · may be incorrectBasalt typically has Mohs hardness 6–7, dark gray to black color, dull to subvitreous luster, very fine interlocking crystals, and no prominent cleavage; density is about 2.8–3.0 g/cm³. The visible pale vein may be quartz or calcite.
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Physical properties
Basalt typically has Mohs hardness 6–7, dark gray to black color, dull to subvitreous luster, very fine interlocking crystals, and no prominent cleavage; density is about 2.8–3.0 g/cm³. The visible pale vein may be quartz or calcite.
Formation & geological history
Basalt forms when mafic lava cools rapidly at or near Earth’s surface, commonly in volcanic fields, oceanic crust, and dikes; its age may range from modern to billions of years. The specimen appears weathered and naturally fractured.
Uses & applications
Used extensively as crushed aggregate, road base, railroad ballast, riprap, concrete aggregate, and dimension stone; attractive polished basalt is occasionally used decoratively. Ordinary fragments have little gem value.
Geological facts
Basalt is the most common volcanic rock and makes up much of the oceanic crust. Andesite is a possible alternative if the rock contains more silica and lighter minerals than typical basalt.
Field identification & locations
Identify it by its dark color, fine texture, heavy feel, and lack of visible large crystals; a streak test is usually gray, and dilute acid should not fizz unless the vein is calcite. Common worldwide, especially near volcanoes and basaltic lava flows.
